The invention relates to a process and apparatus for the purification of crude gases from a carbonaceous feedstock gasification with simultaneous production of: (1) synthesis gas, (2) CO.sub.2 -enriched fuel gas suitable for the generation of electrical energy, e.g., by means of a gas turbine, and (3) H.sub.2 S/COS-rich gas suitable for producing sulfur in a Claus unit. After conversion to the desired H.sub.2 to carbon oxide ratio, the crude gas is cooled and, subsequently, H.sub.2 S and COS are removed therefrom in a lower section of a scrubbing column using a scrubbing medium previously loaded with CO.sub.2. In an upper section of the scrubbing column, CO.sub.2 is scrubbed from the crude gas. Purified synthesis gas is discharged from the scrubbing column.
In the gasification of carbonaceous feedstock (e.g., coal, oil, refinery residues), a gaseous mixture is formed comprising preferably H.sub.2 and CO, but also containing other components, such as CO.sub.2, H.sub.2 S and inert gases.
In correspondence with further usage of the gaseous mixture as synthesis gas (e.g., NH.sub.3 or methanol synthesis), as a starting compound for obtaining H.sub.2 or CO, or as a fuel gas, the hot crude gas, in conventional methods, is first converted in accordance with the marginal range of conditions yielding the required synthesis gas. After conversion, the gas is partially cooled--either in a waste heat boiler for steam generation or by water quenching--and then purified in a two-stage, sour gas scrubbing column. H.sub.2 S and COS are removed in a lower column section while CO.sub.2 is scrubbed out to the required purity in an upper section. Normally, physical scrubbing techniques are utilized for removal of CO.sub.2, H.sub.2 S and COS.
A conventional process for the simultaneous production of synthesis gas and fuel gas, the latter being used for generating electrical energy by means of gas turbines, is disclosed in DE-OS 3,427,633. In this process, two crude gas streams are cooled separately. Only the crude gas stream intended for the production of synthesis gas is converted (i.e., reaction of CO with steam to produce H.sub.2 and CO.sub.2). The gas streams are subsequently scrubbed in two parallel scrubbing columns. During this process, the synthesis gas is first selectively desulfurized in an H.sub.2 S scrubbing step and subsequently subjected to a CO.sub.2 scrubbing step.
In the conventional process, the excess, CO.sub.2 -loaded scrubbing agent from the synthesis gas scrubbing stage is utilized for desulfurization of the fuel gas. By virtue of this combination, it is possible to strip out a portion of the CO.sub.2, that had been scrubbed out in the synthesis scrubbing stage, and to combine it with the fuel gas without any significant additional expenditure of energy. By this procedure, a savings in compression energy is realized in the subsequent admixture of air with the fuel gas inasmuch as less excess air is required to be mixed with the fuel gas in order to limit the maximum combustion temperature in the gas turbines.
However, despite the advantage of CO.sub.2 enrichment in the fuel gas, the conventional process exhibits several substantial drawbacks.
In the H.sub.2 S scrubbing step, in addition to H.sub.2 S and COS, a large amount of CO.sub.2 is scrubbed out as well (in correspondence with the CO.sub.2 partial pressure and CO.sub.2 solubility). In the usual selective sour gas scrubbing operations, H.sub.2 S and COS are concentrated in a downstream enrichment column to such an extent that the H.sub.2 S fraction obtained during hot regeneration of the scrubbing medium can be reacted to elemental sulfur in a Claus unit.
However, in the enrichment column, a cold scrubbing medium already loaded with CO.sub.2 is needed for retaining H.sub.2 S and COS. The cold. CO.sub.2 -loaded scrubbing medium is used to rescrub the H.sub.2 S released together with the CO.sub.2 and so as to deliver a sulfur-free residual gas from the H.sub.2 S enrichment column. Normally, the excess scrubbing medium for the CO.sub.2 scrubbing operation is utilized for this rescrubbing step. However, in the combination disclosed in the above-described conventional method of DE-OS 427,633, there is no available CO.sub.2 -loaded scrubbing medium. Enrichment of H.sub.2 S and COS is therefore impossible. Due to this problem, the sulfur content of the sulfur fraction drops to such a great extent that a Claus unit cannot be employed. Although H.sub.2 S from such a gaseous mixture can be reacted to elemental sulfur by means of an oxidative scrubbing step, these scrubbing operations have the disadvantage that COS does not react and thus cannot be scrubbed out. Due to the high COS content of unconverted crude gas (about 4-5% of the sulfur is generally present in the form of COS), significant environmental pollution problems are encountered.
Normally, electric current generation is adapted to demand. Therefore, fluctuations occur on a daily and seasonal basis. For this reason, the amount of fuel gas needed for the generation of electrical energy produced by coal gasification and the like is likewise variable. To avoid load fluctuations of gasification and of the air fractionator, it is normally desirable to operate gasification at a constant rate and to absorb quantitative fluctuations of the required fuel gas in the synthesis gas production. As a prerequisite of this, the quenching, conversion, and the two parallel scrubbing operations must operate under fluctuating marginal conditions, requiring a high expenditure for control means for the operation of the total installation.
Both lines (gas cooling, conversion, and both sour gas scrubbing stages) must be designed for the respectively maximum amount of gas. Therefore, both lines are oversized and normally do not operate at optimum operating conditions. At the same time, over-dimensioning entails higher initial investment costs.